Very low bitrate and long range radio frequency communication systems are becoming more prevalent. In fact, this long range expresses the capacity of the radio system to sustain greater attenuation than is customary in the radio channel, which requires increased sensitivity of the receiver.
To achieve these high sensitivities, it is usually proposed to reduce the bandwidth of the radio signal as much as possible, the cost being a very low nitrate. The lover the bandwidth, the lower the noise power collected by the receiver, while the energy of the signal concerned remains the same.
As the disadvantage is a particularly low information nitrate, this approach is reserved for transmitters of very low cost and complexity which transmit only very small amounts of data. For example, these typically are autonomous sensors spread across an environment to be monitored (such as objects connected in a LoRaWan® or Sigfox® network, or GPRS/3G, LTE/4G, or other network) or elements of an “Internet of Things”.
The logical consequence of the reduction in bandwidth of the radio channel is its evolution into a model without frequency selection. This choice can result in deep fading phenomena affecting the entire radio signal. To prevent this phenomenon from significantly altering the radio link, it is important that the operators size the link budget of the radio link to have a significant operating margin, which limits the effective range of these networks accordingly.
Spread spectrum techniques, and typically frequency hopping, have been developed to combat this exact phenomenon which is typical of narrowband channels. In general terms, frequency hopping consists of sequentially transmitting/receiving a signal on multiple distinct successive frequencies. A time sequence of successive frequency hops is defined, at the transmitter and at the receiver.
However, the use of this technique requires a high level of performance (in terms of stability and speed of oscillators) and a very good quality of synchronization from the radio frequency stages of the terminals, which is both complex electronically and expensive in terms of energy consumption.
These techniques thus remain out of reach for the most limited devices.
The present invention improves the situation.